A conventional double sealed valves generally type include a valve seat in a communicating passage, which is formed between an upper passage and a lower passage. The valve also includes a first vertical valve stem and a second vertical valve stem. The first stem has a first valve plug for vertically slidable engagement with an inner peripheral surface of the seat through a primary annular packing member, which is fitted to the first plug. The second stem has a second valve plug for compressive engagement with an upper surface of the seat through a secondary annular packing member, which is fitted to the second plug. The plugs can be opened and closed by properly driving the stems with a valve drive mechanism. Each of the packing members is made of rubber, and held in the annular groove formed in the surface of the associated packing member which faces the seat.
The downward movement of the second valve plug brings the secondary annular packing member into compressive engagement with the upper surface of the valve seat. Therefore, when the second plug closes, only a compressive force acts on the secondary packing member, and no shear force acts. On the other hand, the vertical movement of the first valve plug brings the primary annular packing member into axially slidable engagement with the inner surface of the seat, which may be made of stainless steel. Since a large frictional resistance acts on the sliding surfaces, a shear force is produced at the boundary between the outer or exposed part of the primary packing member which protrudes from the associated groove and the remaining inner part in the groove. If the shear force is excessive, the packing member may be damaged.
When the valve is closed, the annular packing members are restrained and compressed by valve seat surfaces. When the valve opens, the packing members are freed from restraint, so that their outer parts swell or dilate. The dilatation of the outer parts may increase remarkably with the pressure fluctuation and/or temperature change of the liquid flowing through the valve when the valve is open. If the outer part of the primary packing member has greatly swelled when the first valve plug transfers or shifts from its open state to its closing action, the forced engagement of the outer part with the inner surface of the seat applies an excessive shear force to the outer part. The outer part may consequently be damaged in a short time.
The ratio of the volume of the outer part to that of the inner part is relational to the dilatation of the outer part due to the presence fluctuation and/or temperature change of the liquid flowing through the valve. The larger in volume the inner part is as compared with the outer part, the smaller the dilatation of the outer part is. In addition, if the outer part of the primary annular packing member is partially restrained when the first valve plug is open, the outer part can be restrained from swelling when the plug opens.